This invention relates to thermoplastic substrates covered with a protective coating. More particularly, it relates to improved silicone resin coating compositions for substrates which, after curing, may be thermoformed without showing cracks or adhesion failure. The thermoformable top coat forms an adherent protective, abrasion-resistant layer on the substrate.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles such as trains, buses, taxis and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch due to everyday contact with abrasives such as dust or cleaning equipment. Marring results in impaired visibility and poor aesthetics, and often requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature and good dimensional stability. It is also self-extinguishing and easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
To improve the abrasion resistance of plastics, mar-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, have been developed. U.S. Pat. Nos. 3,708,225 (Misch et al.), 3,986,997 (Clark), 4,027,073 (Clark), 4,159,206 (Armbruster et al.) and 4,177,315 (Ubersax), for example, describe such compositions. Improved such compositions are described in commonly assigned copending U.S. Application Ser. No. 964,910, filed Nov. 30, 1978 (now abandoned) and U.S. Pat. Nos. 4,277,287 (Frye) and 4,309,319 (Vaughn, Jr.).
Silicone resin coatings have been additionally improved in recent years by the addition of compounds which screen or absorb ultraviolet radiation and thereby serve to prolong the service life of coated articles. Such coatings and compounds are described, for example, in U.S. Pat. Nos. 4,299,746 (Frye) and 4,278,804 (Ashby et al.), 4,373,060 (Ching), 4,373,061 (Ching), and 4,374,674 (Ashby et al.)
In order to promote adhesion of silicone resin coating compositions to thermoplastic substrates such as Lexan.RTM. or polymethylmethacrylate, primer coats are often necessary. U.S. Pat. No. 4,188,451 (Humphrey), for example, discloses the UV-cured reaction product of a polyfunctional acrylic ester and an unsaturated radical-containing silane as a primer, and the aforementioned U.S. Pat. No. 4,309,319 discloses the use of a thermosetting acrylic resin. The aforementioned Ching patents, on the other hand, disclose UV screen-functionalized compositions which do not need primers and instead adhere directly to polycarbonate substrates without pre-treatment (other than routine cleaning), although an aging period is required after preparation of the compositions before they are ready to use.
All of the above-mentioned patents and applications are incorporated herein by reference.
Two disadvantages to the abrasion-resistant silicone resin coating compositions currently used to coat polycarbonate are that they are not thermoformable after curing, especially in thick applications, and primerless coatings must be aged, as mentioned previously. Poor thermoformability means that bending or working a coated polycarbonate substrate will often lead to cracking or crazing of the silicone coating. As a consequence, articles must be coated after forming, which may entail time delays and shipment of uncoated articles which may be inadvertently abraded in transit. The aging period required for the aforementioned primerless coating compositions entails warehousing the prepared compositions, the coats of which may outweigh any disadvantages in using primer coats.
It has now been discovered that the thermoformability of the UV screen-functionalized compositions described by Ching in U.S. Pat. Nos. 4,373,060 and 4,373,061 can be improved and their aging time substantially reduced by the addition of a small amount of a Lewis acid compound, such as ferric chloride, aluminum chloride, zinc chloride, and the like. Thermoformability and other physical properties are also improved when those coating compositions are used in conjunction with thermoplastic acrylate primers. Use of a Lewis acid compound has been found to assist the incorporation of silylated UV screens into the structure of silicone resin coating compositions and to improve their thermoformability. Employing extensible acrylate primers with the above-mentioned silicone resins, which are disclosed to be primerless coating compositions on polycarbonate, results in a dual component coating system which is thermoformable after curing.